Updated the hard app reset instructions.

Completed the list of tables to remove to fully reset the app settings.
See also: #2383

Signed-off-by: Matěj Týč <matej.tyc@gmail.com>

doc/developer.md

@@ -19,4 +19,7 @@ DROP TABLE oc_mail_accounts;
 DROP TABLE oc_mail_aliases;
 DROP TABLE oc_mail_coll_addresses;
 DROP TABLE oc_mail_attachments;
-```
+DROP TABLE oc_mail_mailboxes;
+DROP TABLE oc_mail_messages;
+DROP TABLE oc_mail_recipients;
+```